Bound `BroadcastState.cancelled` entries to receiver progress,
terminal state and resource lifetime instead of retaining completed
`Task`s indefinitely.
Deats,
- make EOC durable so awakened peers never re-enter a closed source.
- close root broadcasters during explicit `MsgStream` and
`LinkedTaskChannel` teardown without re-entrant EOC closure or
breaking `MsgStream.aclose()` overrides.
- reject non-positive fan-out retention capacity before constructing
an unusable zero-length queue.
- cover child/root cancellation cleanup, terminal peer wakeups,
wrapper teardown, subclass compatibility and zero-buffer rejection.
Prompt-IO: ai/prompt-io/opencode/20260813T181901Z_a2e0df4b_prompt_io.md
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi